Inductive write heads are used for recording information on magnetic media, such as magnetic disks. The recorded information can be read out by an inductive read/write head. Alternatively, MR heads can be used for sensing signals which have been recorded on a magnetic medium. The signal sensed by an MR head is proportional to the magnetic flux associated with the recorded signal, and not to the rate of change of flux which is sensed by an inductive head. Thus an MR head can detect a magnetic field representing a recorded signal without any relative motion between the storage medium and the MR head.
A typical thin film MR head incorporates a single MR element, preferably made of a layer of Permalloy having an easy axis of magnetization. During operation of a data storage apparatus, such as a disk drive, an electric sense current is directed to the MR element. The magnetic field being sensed exerts a torque on the magnetic moment in the MR thin film causing a change in the resistivity of the film. The change in resistivity is proportional to the strength of the field being measured and causes variations in the resistance of the MR element. Detection of such variations provides a readout signal related to the data signal recorded on the magnetic medium.
FIG. 1 is a cross-sectional view of an embodiment of a prior art magnetoresistive (MR) transducer taken along a plane parallel to the air bearing surface (ABS) of the device. The MR transducer, designated by reference numeral 2, comprises a tri-layer structure 4. Included in the tri-layer structure 4 is a spacer layer 8 sandwiched between a magnetoresistive layer 6 and a soft magnetic adjacent layer 10. The magnetoresistive layer 6 is normally made of a soft magnetic material, such as Permalloy which is an alloy of nickel and iron, having a high permeability and a low coercive force. During the read process, changes in magnetic flux passing through the magnetoresistive layer 6 correspondingly vary the resistivity of the magnetoresistive layer 6. As is well-known in the art, the magnetoresistive layer 6 must be aligned in a single domain-state to suppress the Barkhausen noise. Hard magnetic layers 12 and 14 disposed at the end portions of the tri-layer structure 4 fulfill this function by cooperatively providing a longitudinal magnetic bias for magnetic domain alignment. Moreover, for magnetoresistive layer 6 to operate within a linear region, another bias, called the transverse magnetic bias, must also be applied to magnetoresistive layer 6. The soft adjacent layer 10 carries out this duty by providing the required magnetic bias.
Hard magnetic layers 12 and 14 are permanently magnetized and are disposed in direct contact with the end portions of the magnetoresistive layer 6. Hard magnetic layers 12 and 14 supply the longitudinal magnetic bias to magnetoresistive layer 6 through a process called magnetic coupling. The soft adjacent layer 10, generally made of a soft magnetic material having a high permeability, a low coercive force and a high resistivity, branches out a fraction of the bias current applied across electrical leads 16 and 18 during normal operations. The branched out current induces a magnetic flux which traverses the magnetoresistive layer 6 as the transverse magnetic bias.
During the read mode, the bias current applied across electrical leads or conductors passes through the magnetoresistive layer 6 via the hard-magnetic layers 12 and 14. Changes in the magnetic flux intercepted by the transducer 2 vary the electrical resistivity of the magnetoresistive layer 6. The bias current flowing through the magnetoresistive layer 6 with varying resistivity accordingly generates a varying voltage. The varying voltage corresponds to the information read out from the storage medium (not shown). Transducers of this type are described in U.S. Pat. No. 4,639,806, entitled "Thin Film Magnetic Head Having a Magnetized Ferromagnetic Film on the MR Element", Kira et al., issued Jan. 27, 1987.
FIG. 2 shows another type of prior art magnetic transducer designated by reference numeral 20 which is quite similar in structure to transducer 2 shown in FIG. 1. However, there are underlayers 22 and 24 disposed between hard magnetic layers 12 and 14, respectively, and the end portions of magnetoresistive layer 6. Underlayers 22 and 24 are typically made of a non-magnetic material and serve to prevent magnetic coupling between hard-magnetic layers 12 and 14 and magnetoresistive layer 6. Underlayers 22 and 24 also provide the function of preserving a desirable orientation of the crystalline structure of the hard magnetic layers 12 and 14 during the fabrication process. The required longitudinal magnetic bias to the magnetoresistive layer 6 is supplied by hard magnetic layers 12 and 14 through a process called magnetostatic interaction. Transducers of this type are described in U.S. Pat. No. 5,005,096, entitled "Magnetoresistive Read Transducer Having Hard Magnetic Shunt Bias", Krounbi et al , issued Apr. 2, 1991.
To further improve the longitudinal magnetic bias, a different type of transducer is devised. FIG. 3 shows such a prior art transducer designated by reference numeral 26. In transducer 26, hard-magnetic bias layers 28 and 30 form abutting contacts with magnetoresistive layer 6 through abutting junctions 32 and 34, respectively. Hard-magnetic layers 28 and 30 provide a more continuous longitudinal magnetic bias to the magnetoresistive layer 6, in comparison with the transducers 2 and 20 shown in FIGS. 1 and 2, respectively. It should be noted that the thickness t.sub.h of hard magnetic layers 28 and 30 is comparable in dimension with the corresponding thickness t.sub.s of the tri-layer structure 4'. As a consequence, transducer 26 also realizes another important advantage, namely, the overlying magnetic shield and the dielectric layer (not shown) can be deposited atop the transducer 20 with improved step coverage. That is, the aforementioned overlying layers can be deposited with less steep steps, thereby minimizing the chance of generating electrical shorts between the electrical leads 16 and 18 and the upper magnetic shield layer. There are a number of operational similarities between transducer 26 and transducers 2 and 20. As with the transducer 20 described above, non-magnetic underlayers 36 and 38 can be disposed between the respective hard-magnetic layers 28 and 30 and the magnetoresistive layer 6. In this case, hard magnetic layers 28 and 30 provide the longitudinal magnetic bias via the process of magnetostatic interaction. Optionally, underlayers 36 and 38 can be removed such that hard-magnetic layers 28 and 30 provide the longitudinal magnetic bias mainly through the process of magnetic coupling. In reality, with the underlayers 36 and 38 installed and depending on their thicknesses, both the process of magnetostatic interaction and the process of magnetic coupling contribute proportionally to the magnetic bias. An intermediate biasing scheme can always be arranged by manipulating the thicknesses of the underlayers 36 and 38 such that neither the process of magnetostatic interaction nor the process of magnetic coupling take dominance. Transducers of this type are described in U.S. Pat. No. 5,018,037, entitled "Magnetoresistive Read Transducer Having Hard Magnetic Bias", Krounbi et al., issued May 21, 1991.
A major operational problem of the transducers 2, 20 and 26 as described, is the inadequacy of longitudinal bias provided by the respective hard-magnetic bias layers to the magnetoresistive layer 6. This can best be explained by referring to FIG. 4 which shows the hysteresis characteristics of various magnetic materials. As mentioned earlier, magnetoresistive layer 6 is made of a soft magnetic material having a high permeability and a low coercive force. The magnetic behavior of such a material is represented by a hysteresis curve 40 shown in FIG. 4. Similarly, hard magnetic material suitable for hard-magnetic bias layers 12, 14, 28, and 30 generally comprises a high coercive force ideal for sustaining a high magnetic moment permanently. Another hysteresis curve, designated by reference 42, represents the magnetic behavior of such material. As graphically illustrated in FIG. 4, hysteresis curve 40 has a relatively low coercive force H.sub.cs and a high remanent magnetization M.sub.rs. These characteristics enable the soft magnetic material, which is the material normally used for the magnetoresistive layer 6, to react swiftly in response to external magnetic flux changes. On the other hand, hysteresis curve 42 includes a high coercive force H.sub.ch and a comparatively low remanent magnetization M.sub.rh. Such attributes are ideal for hard magnetic layers such as layers 28 and 30 which require stronger magnetizing force to be magnetized, but once they are magnetized, also require strong opposite magnetizing force for demagnetization. However, with the current state of the art, among the various materials available in thin-film technology, the remanent magnetization M.sub.rh of hard magnetic material 42 is inherently less than the corresponding remanent magnetization M.sub.rs of soft magnetic material 40, as is depicted in FIG. 4. The difference in remanent magnetization between the hard magnetic layers, such as layers 28 and 30 shown in FIG. 3, and the tri-layer structure, such as structure 4', is even more pronounced due to the contributory factor by the soft adjacent layer, such as layer 10.
Reference is now directed back to FIG. 3. Magnetic flux emerging out of hard magnetic bias layer 28 into layer 30 via the tri-layer structure 4 is essentially the mathematical product of the remanent magnetization M.sub.rh of hard magnetic layer 28 and the area of abutting junction 32. However, for magnetoresistive layer 6 to be magnetically saturated fully along its easy axis 44 within a single domain state, the required magnetic flux is approximately the mathematical product of remanent magnetization M.sub.rs of soft magnetic layer 6 and the area of abutting junction 32. As mentioned previously, remanent magnetization M.sub.rh of hard magnetic bias layers 28 and 30 is less than the remanent magnetization M.sub.rs of magnetoresistive layer 6. With the shared abutting junction 32, longitudinal bias for the magnetoresistive layer 6 is bound to fall short of the necessary requirement. As a result, the magnetoresistive layer 6 may not be properly aligned in a single domain state so that the transducer 26 is subject to considerable Barkhausen noises during operation.
To rectify this problem, the thickness t.sub.h of hard magnetic layers 28 and 30 can be increased for garnering additional magnetic flux. In that case, the transducer 26 needs to be fabricated with bulging end portions having thicknesses t.sub.h larger than the corresponding thickness t.sub.s of the tri-layer structure 4'. Transducer 26 thus fabricated may result in a physical shape not much different from transducers 2 and 20 as shown in FIGS. 1 and 2, respectively. The original purpose of providing a more continuous longitudinal bias to magnetoresistive layer 6 through abutting junctions 32 and 34 is thereby defeated. In addition, transducers with large thicknesses are not capable of reading recorded media having high linear recording density.